Inadequate delivery and tumor spread after systemic administration remains a major -unsolved- obstacle that limits the efficacy of systemically administered oncolytic viruses for the treatment of cancer. This is a significant, rate limiting step for the pre-clinical and clinical development of these agents. The tumor stroma (tumor endothelium, tumor fibroblasts, and extracellular matrix) contributes to the poor efficacy of oncolytic agents, as it creates a barrier for viral entry and replication in tumor cells in vivo. Our long term goal is to improve the efficacy of systemically administered oncolytic viruses by understanding the interactions among virus, tumor and stroma, and exploiting them therapeutically. The overall objective of this application, which is the next step in the pursuit of that goal, is to characterize the specificity and therapeutic effects of tumor stromal and vascular targeting of oncolytic measles viruses via the urokinase receptor (with a virus called MV- uPA), a critical tumor/stromal cell surface glycoprotein. The central hypothesis of this application is that selective viral uPAR targeting of tumor stroma/vasculature will significantly enhance the virus' effects against primary tumors and metastases. The rationale for the proposed research is that understanding of how oncolytic viruses interact with important components of the tumor stroma will help create smarter, safer oncolytic viral vectors with a better therapeutic window. The proposed research is relevant to NIH's mission, in that it pertains to developing safer, more effective biological agents for the fight against cancer. Guided by strong preliminary data, the central hypothesis will be tested by pursuing three specific aims: 1) Characterize MV-uPA's tumor selectivity and virus-host interactions in syngeneic murine cancer models; 2) to further characterize the in vivo effects of tumor and stromal urokinase receptor targeting by MV-uPA in breast cancer models; and 3) to characterize the tumor vascular targeting abilities of MV-uPA and its contribution to the virus' overall anti-tumor effects. Aim 1 will investigate the tumor selectivity, viral replication and organ biodistribution, of uPAR retargeted measles viruses, after systemic administration, using molecular and immunohistochemical techniques. In addition, we will characterize the safety and efficacy dose response to the virus in immunocompetent mammary and colon cancer models. In aim 2, we will characterize the antimetastatic activity of uPA retargeted measles virus using, in vivo imaging techniques. We will also characterize MV-uPA's ability to target stromal cells, and its consequences in regards to antitumor efficacy. In aim 3, we will characterize the kinetics of virus/endothelial interactions as they pertain to the virus' potential antiangiogenic effects, and the ability of the uPAR targeted viruses to enhance viral delivery into breast tumor in vivo via tumor endothelium. The approach is innovative and significant, because it will bring new knowledge on in vivo interactions between virus and stroma, which will translate into the development of better oncolytic viral agents for the benefit of patients with cancer.